Large private communication networks often resemble the public communication network in their architecture: they comprise (a) a plurality of "local" switching systems--typically private branch exchanges (PBXs)--which are connected to different groups of customer terminals and provide communication services for those terminals, and (b) "tandem" switching systems--typically also PBXs--whose principal function is to provide communication connections between the "local" switching systems. It is also typical that a "tandem" switching system provides connections between "local" switching systems within only a single, limited, geographical area, whereas connections between geographically widely-dispersed switching systems--both "local" and "tandem" are provided by public network long-distance carriers. The carriers also provide connections between the private networks and the public network.
An example of carrier equipment that provides the connection services mentioned above is the AT&T digital access cross-connect system (DACS). Located at a carrier service node, the DACS is connected by trunks to both "local" and "tandem" private-network switches, and to the carrier's switch for public network access. The DACS provides relatively static interconnections between trunks which are connected thereto, as opposed to dynamic, switched, interconnections. A call incoming to the DACS over a first trunk from an originating "local" switch of a private network is connected through the DACS to the private network's "tandem" switch over a second trunk. The "tandem" switch performs digit collection on the call to determine the call destination. The "tandem" switch has routing information pertaining to the private network, which enables it to determine where and how the call should be routed to its destination. Based on that routing information, the tandem switch selects a third trunk leading back to the DACS, and connects the call thereto. The call is in turn connected through the DACS, to a trunk leading to a destination "local" switch of the private network. The call path from the originating to the destination "local" switches is thus established through the DACS and the "tandem" switch, and requires four trunks.
Economics dictate a desire to reduce the number of trunks used in establishing the call path. For example, the greater the number of trunks used for each call path, the greater the cost to the owner of the private network of: (a) buying, installing, and maintaining owned trunks, (b) renting leased trunks, and/or (c) accessing public network trunks. Hence, the owner of a private network desires to use as few trunks per call as possible. It is therefore to the competitive advantage of a carrier to provide the private network owner with the capability of reducing the number of trunks required for a call.